As power-electronic (PE)-based systems become increasingly common in the electric power grid, the insulation systems used in medium- and high-voltage (HV) applications will be exposed to high-frequency (HF) electric fields. Therefore, the insulation materials must be characterised using HF waveforms. However, generating these waveforms presents a significant challenge due to the large reactive power associated with the d (Formula presented.) /d (Formula presented.). This paper proposes a resonant test system with a ferrite-based transformer for HF insulation testing. The resonant circuit is formed by the transformer's leakage inductance and the insulation sample capacitance, with an adjustable frequency tuning capacitor. The system can be driven with an inverter or linear power amplifier. Increasing the test voltage level while maintaining the same test frequency presents several challenges: transformer core grounding, high resonant current and implications for bobbin and insulation design. This paper investigates these challenges and proposes an oil-insulated resonant transformer, capable of extending the test voltage to 23 kV_pk for HF insulation tests at around 40 kHz. High-frequency breakdown tests are performed on enamelled copper wire in various insulation media using the prototype resonant test system, highlighting the importance of the dielectric's thermal performance.